(Updates with Ukraine statement on power plant captured by Russians; U.S., Russian diplomats to have phone call)
* Russian-backed forces claimed takeover of Vuhlehirsk plant
* Russia cut back gas flows to Europe in energy stand-off
* Polish PM says Russia cannot be trusted on grains deal
* Russia in 'massive redeployment' in south, Ukraine says
By Natalia Zinets
KYIV, July 27 (Reuters) - Russian forces have taken over Ukraine's second biggest power plant, an adviser to President Volodymyr Zelenskiy said on Wednesday, following an earlier claim by Russian-backed forces to have captured it intact.
Seizing the Soviet-era coal-fired Vuhlehirsk power plant in eastern Ukraine would be Moscow's first strategic gain in more than three weeks in what it calls its "special operation" to demilitarise and "denazify" its neighbour.
"They achieved a tiny tactical advantage - they captured Vuhlehirsk," adviser Oleksiy Arestovych said in an interview posted on YouTube.
Russia was undertaking a "massive redeployment" of troops to three southern regions, Arestovych said.
U.S. Secretary of State Antony Blinken, whose government describes Russia's invasion as an unprovoked war of aggression, said on Wednesday that he planned a phone conversation with Russian foreign minister Sergei Lavrov - the first between the two diplomats since before the start of the war.
The call would not be "a negotiation about Ukraine," Blinken said at a news conference, restating Washington's position that any talks on ending the war must be between Kyiv and Moscow.
Russia has received no formal request from Washington about a phone call between Blinken and Lavrov, TASS news agency reported.
The United States has made "a substantial offer" to Russia for it to release U.S. citizens WNBA star Brittney Griner and former U.S. Marine Paul Whelan, Blinken said, without providing details.
Aside from discussing Americans detained by Russia, Blinken said he would raise with Lavrov the tentative deal on grain exports reached last week between Russia, the United States, Turkey and Ukraine.
Rising energy prices and a global wheat shortage that threatens millions in poorer countries with hunger are among the far-reaching effects of the war.
GAS FLOWS
Russia reduced gas flows to Europe on Wednesday in an energy stand-off with the European Union. It has blocked grain exports from Ukraine since invading on Feb. 24, but on Friday agreed to allow deliveries through the Black Sea to Turkey's Bosphorus Strait and on to global markets.
The deal was almost immediately thrown into doubt when Russia fired cruise missiles at Odesa, Ukraine's largest port, on Saturday, just 12 hours after the deal was signed.
"The day after the signing (of the agreement), the Russian armed forces ... attacked Odesa," Polish Prime Minister Mateusz Morawiecki told reporters in Warsaw. "It follows that such agreements cannot be considered fully credible, because unfortunately that is what Russia is like."
Before the invasion and subsequent sanctions, Russia and Ukraine accounted for nearly a third of global wheat exports.
Russian and Russian-backed forces have been struggling to make meaningful progress on the ground since their capture in early July of the eastern Ukrainian city of Lysychansk.
They have been repeatedly pushed back by fierce Ukrainian resistance.
Unverified footage posted on social media appeared to show fighters from Russia's Wagner private military company posing in front of the Vuhlehirsk power plant, which some Russian state media - citing Russian-backed officials - reported separately had been stormed.
One of the Wagner fighters in front of the plant showed his watch to the camera - the time on it was 1001 local and gave the date as July 26.
Reuters could not immediately verify the video or whether the plant had switched to Russian control.
The same unverified footage showed that working parts of the Soviet-era power plant, which is perched on the shore of a huge reservoir, appeared to be undamaged.
RUSSIAN SETBACK IN KHERSON
British military intelligence said on Wednesday that Wagner fighters had probably succeeded in making tactical advances in the Donbas region of eastern Ukraine around the power plant and the nearby village of Novoluhanske.
It said some Ukrainian forces had probably withdrawn from the area.
Russian forces meanwhile suffered a setback in southern Ukraine's Kherson region after Ukrainian forces struck an important bridge straddling the Dnipro river with what a Russian-appointed local administrator said were U.S.-supplied high mobility artillery rocket systems (HIMARS).
The Antonivskyi bridge is the city of Kherson's sole span across the river. Kirill Stremousov, deputy head of the Russian-appointed city administration, told Russia's RIA news agency it had been closed to traffic after the strike.
He said Russia was ready to compensate for it being taken out of action with pontoon bridges and ferries.
In a Wednesday evening address, Zelenskiy said Ukraine would rebuild the Antonivskyi bridge and other crossings in the region.
"We are doing everything to ensure that the occupying forces do not have any logistical opportunities in our country."
Russia was concentrating "the maximum number of troops" in the direction of Kherson, Oleksiy Danilov, secretary of Ukraine's National Security and Defence Council, said on Twitter but gave no details.
Ukraine has spoken of launching a major counter-offensive in the south to try to retake cities such as Kherson. Rendering the bridge unusable for Russian forces is seen by Western military analysts as something that would make it much harder for Moscow's forces to operate smooth supply lines and defend land they have seized.
(Reporting by Reuters bureaux; Writing by Andrew Osborn, Nick Macfie and Grant McCool; Editing by Mark Heinrich, Catherine Evans and Howard Goller)
